The invention pertains to the field of electronic reproduction technology and relates to a method of determining the area coverage of printing plates. The area coverage is the proportion of the area of a printing plate that is covered with printing ink during printing, which ink is, then, transferred to the printing material (e.g., paper). For an image, the area coverage is given by the proportion of the area that is covered by the halftone dots, in relation to the total area of the image. For a text, the area coverage is given by the proportion of the area that is covered by the lines and curves of the letters, in relation to the total area of the text block. Global and local area coverages, determined strip by strip, are needed for presetting the inking zones in the inking unit of an offset printing press. For the inking zones divided up over the width of the printing plate, the quantity of printing ink fed in must be set such that it corresponds to the quantity of printing ink taken off the corresponding strip on the printing plate, which, in turn, depends on the area coverage of the printing plate in the relevant strip, that is to say, on the size of the halftone dots, the thickness of the lines of the text and so on.
In reproduction technology, printing originals for printed pages are produced that contain all the elements to be printed, such as texts, graphics, and images. For colored printing, a separate printing original is produced for each printing ink, which contains all the elements that are printed in the respective color. For four-color printing, these are the printing inks cyan, magenta, yellow, and black (CMYK). The printing originals, separated in accordance with printing inks, are also referred to as color separations. The printing originals are generally scanned and exposed by an exposer onto films, which are, then, processed further in order to produce printing plates for printing large editions. Alternatively, the printing originals may also be exposed directly onto printing plates in special recorders, or they are output directly to a digital printing press as digital data.
According to the prior art, the printing originals are reproduced electronically. Here, images are scanned in a color scanner and stored in the form of digital data. Texts are produced by text processing programs and graphics by symbol programs. Using a layout program, the image, text, and graphics elements are assembled to form a printed page. Following separation into the printing colors, the printing originals are, then, present in digital form. Nowadays, the page description languages Postscript and Portable Document Format (PDF) are largely used as data formats for describing the printing originals. Before the recording of the printing originals, such a description is converted into a description in which the image points are each assigned a gray value if the printed page is printed in black and white or, respectively, the image points are each assigned color values if the printed page is a colored print. If the printed page is printed in the four printed colors CMYK, then the amounts of ink to be printed in each image point are described by four color values. For example, four bytes are produced for each image point, corresponding to the four color values. The color values, then, have one of 256 possible steps between 0 and 255. The printing-original data for a printing color can, therefore, also be understood as a xe2x80x9cblack and white page,xe2x80x9d whose xe2x80x9cgray valuesxe2x80x9d specify the amount of the associated printing color in each image point.
In the printing press, printing ink in the form of a thin layer is applied to the printing material by the printing points on the printing plate. The task of the inking unit is to supply the printing points continuously with fresh color so that the printing process can be maintained. The quantity balance between feed and discharge of ink must be balanced if color density fluctuations in the printed image are to be avoided. In addition to the balanced quantity balance, the constancy of the ink layer thickness on the printing points of the printing plate or the printed points on the printing material is of critical importance for the printing quality. To ensure the constancy, in an offset printing press, the feed of the printing ink is set zone by zone with an inking zone control system. The printable width of the printing press is subdivided into inking zones, for which the quantity of ink fed in can be controlled separately. Each inking zone is, for example, 32.5 mm wide. To control the quantity of ink, a resilient ink knife is brought, for example, by zone screws, to a different distance from the ink ductor roll that, as a result, picks up a different quantity of ink from the ink fountain in the individual zones. According to another system, there is an eccentric actuating cylinder in each inking zone, whose distance from the ink ductor roll is set differently in each zone.
To be able to set the actuating devices in the inking zones correctly, it is necessary to know what quantity of ink in the individual inking zones is taken off by the printing plate and transferred to the printing material. This quantity is calculated from the area coverage of the printing plate in the inking zones. The printing plate is subdivided into strip-like regions by the inking zones, that is to say, the area coverages in these strips must be determined. According to the prior art, the print-ready printing plate, before being clamped into the printing press, is scanned in a printing-plate reader to determine the area coverages in the strips that correspond to the inking zones. The values determined are transmitted to the control desk of the printing press, and the actuating devices for the feed of the printing ink are set appropriately from there for the inking zones. A further prior art method derives the area coverages of the inking zones from the printing-original data that are produced during electronic reproduction. Such a process saves the investment costs for a printing-plate reader and expenditure of time for scanning the printing plates before starting printing.
It is accordingly an object of the invention to provide a method of determining the area coverage of printing plates that overcomes the hereinafore-mentioned disadvantages of the heretofore-known devices and methods of this general type and that provides a method of producing coarse color-value image data with which the area coverages for setting the inking zone can even be determined for the case in which various raster systems are used on a printed sheet.
With the foregoing and other objects in view, there is provided, in accordance with the invention, a method for producing a printing plate, including the steps of determining area coverages of the printing plate for controlling quantities of ink in inking zones of a printing press by producing color-value printing data from existing printed-sheet data based upon a standard raster system, converting the color-value printing data into rastered printing data with a raster generator, exposing the rastered printing data onto one of film material and the printing plate with an exposer, utilizing various raster systems with different dot gains on the printing plate, dependent upon a raster system used, applying in the raster generator an associated dot gain correction curve and a linearization correction curve to the color-value printing data, the associated dot gain correction curve describing a change in a dot gain between the standard raster system and the raster system used, the linearization correction curve describing a relationship between raster point sizes in the rastered printing data and the raster point sizes exposed, and determining the area coverages of the printing plate in a unit for area coverage calculation, the unit for area coverage calculation being supplied with coarse color-value image data derived from the color-value printing data, a dot gain correction curve for each raster system used, and correction location data describing which raster system is used in given areas of the printing plate.
With the objects of the invention in view, there is also provided a method for producing a printing plate, including the steps of determining area coverages of the printing plate for controlling quantities of ink in inking zones of a printing press by producing color-value printing data from existing printed-sheet data based upon a standard raster system, converting the color-value printing data into rastered printing data with a raster generator, exposing the rastered printing data onto one of film material and the printing plate with an exposer, utilizing various raster systems with different dot gains on the printing plate, dependent upon a raster system used, applying in the raster generator an associated dot gain correction curve and a linearization correction curve to the color-value printing data, the associated dot gain correction curve describing a change in a dot gain between the standard raster system and the raster system used, the linearization correction curve describing a relationship between raster point sizes in the rastered printing data and the raster point sizes exposed, and determining the area coverages of the printing plate in a unit for area coverage calculation, the unit for area coverage calculation being supplied with coarse raster image data derived from the rastered printing data and the linearization correction curve.
In accordance with another mode of the invention, the correction location data for each image point contains an identifier regarding which raster system is used in the image point.
In accordance with a further mode of the invention, the correction location data is embedded in the coarse color-value image data.
In accordance with an added mode of the invention, the coarse color-value image data is transmitted to a measuring system on the printing press as predefinition values for colors to be achieved in a print.
In accordance with a concomitant mode of the invention, the exposer is integrated into the printing press.
Other features and modes that are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a method of determining the area coverage of printing plates, it is, nevertheless, not intended to be limited to the details shown because various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof, will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.